RED TIDE

Definition
Red tide is a colloquial term used to refer to one of a variety of natural phenomena known as a harmful algal blooms or HABs. The term red tide specifically refers to blooms of a species of dinoflagellate known [8] as Karenia brevis. It is sometimes used to refer more broadly to other types of algal blooms as well. The term red tide is being phased out among researchers for the following reasons: 1. Red tides are not necessarily red and many have no discoloration at all. 2. They are unrelated to movements of the tides. 3. The term is imprecisely used to refer to a wide variety of algal species that are known as bloomformers. As a technical term it is being replaced in favour of more precise terminology including the generic term harmful algal bloom for harmful species, and algal bloom for non-harmful species. The term red tide is most often used in the United States of America to refer to Karenia brevis blooms in the eastern Gulf of Mexico, also called the Florida red tide. These blooms occur almost annually along Florida waters. The density of these organisms during a bloom can exceed tens of millions of cells per litre of seawater, and often discolor the water a deep reddish-brown hue. The term red tide is also sometimes used to describe harmful algal blooms on the northern east coast of the United States, particularly in theGulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. These blooms of organisms cause severe disruptions in fisheries of these waters as the toxins in these organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin.[9]

Causes of red tide
The occurrence of red tides in some locations appear to be entirely natural (algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents)[10][11] while in others they appear to be a result of increased nutrient loading from human activities.[12] The growth of marine phytoplankton is generally limited by the availability of nitrates and phosphates, which can be abundant in agricultural run-off as well as coastal upwelling zones. Coastal water pollution produced by humans and systematic increase in sea water temperature have also been implicated as contributing factors in red tides[citation needed]. Other factors such as iron-rich dust influx from large desert areas such as the Saharan desert are thought to play a major role in causing red tides.[13] Some algal blooms on the Pacific coast have also been linked to occurrences of large-scale climatic oscillations such as El Niño events. While red tides in the Gulf of Mexico have been occurring [14] since the time of early explorers such as Cabeza de Vaca, it is unclear what initiates these blooms and how large a role anthropogenic and natural factors play in their development. It is also debated whether the apparent increase in frequency and severity of algal blooms in various parts of the world is in fact a real increase or is due to increased observation effort and advances in species identification methods.[15][16]